1. Field of the Invention
The present invention relates to a method and apparatus to calibrate a borehole during a drilling operation by taking two measurements of the same formation with two different tools to determine borehole caliber through an iteration process. The tools employed are borehole and lithology sensitive measuring-while-drilling tools.
2. Description of the Prior Art
There have been many devices and methods developed for calibrating a borehole to achieve an accurate profile of the walls of the hole. However, none of these devices have been capable of making the measurements while the drilling operation is being carried out. For the most part, they are wireline devices which are used to log the well hole after the drill string has been withdrawn. Examples of such wireline devices can be found in U.S. Pat. Nos. 2,596,023; 2,596,024; 3,330,374; 3,464,513; 3,517,767; and 3,590,940.
Clearly, there are many advantages which could be achieved by recording the caliper profile of a borehole during the drilling operation. Among the advantages of such a caliper measurement would be to provide warning to the driller that the walls of the borehole are collapsing or closing causing a narrowing or necking of the borehole which could prevent withdrawal of the drill and possibly even gripping the drill string with sufficient force to interrupt the rotary drilling. In a similar way, such a caliper would provide warning that the borehole walls are sloughing in such a manner as to form a large cavity, creating a lot more debris which would necessarily have to be removed and would slow the drilling operation. The driller would then have the option of stabilizing the well before serious damage was done. Another use of a caliper measurement taken while drilling would be to allow borehole correction of formation sensing tools in the same manner as used in wireline logging.
Most wireline tools, because they are not used for some time after a well is drilled, encounter a problem in dealing with the mud cake which begins to form on the borehole walls shortly after drilling. The present invention overcomes this problem by using tools which are in the drill string closely spaced from the drill bit and therefor make their respective measurements prior to development of appreciable mud cake. Further, the present invention does not require physical contact with the borehole wall, as for example, with a six arm caliper or the asymmetrically operated devices which actually penetrate the mud cake. Such mechanical calipers clearly cannot be used while drilling.
There is a limited amount of room available in a drill string for downhole tools. The present invention provides for maximum utilization by employing known formation measurement-while-drilling tools to make measurements which are uniquely utilized to iterate borehole caliber from their respective measurements, without adversely affecting the drilling operation.